This invention relates, in general, to semiconductor devices, and more particularly, to a method of fabricating a semiconductor device in which the active junction is covered with a fritted glass layer that is fired by a rapid heating apparatus.
Surface passivation using glass powder or fritted glass is effective for achieving good reliability in high voltage silicon power devices since glass is electrically stable, easy to coat as a thick film, and is resistant to humidity. Therefore, many glass passivated silicon devices, such as thyristors, transistors, and diodes, have been developed.
Glasses as passivants for silicon devices are classified roughly into zinc-based glass and lead-based glass. The procedure for mixing these types of glasses with photoresist is described in the following references. In U.S. Pat. No. 3,355,291 entitled "Application Of Glass To Semiconductor Devices", borosilicate glass is mixed with a photoresist polymer. U.S. Pat. No. 3,632,434 entitled "Process For Glass Passivating Silicon Semiconductor Junctions", describes a method for mixing a lead oxide compound and a liquid organic.
Mixing the fritted glass and photoresist results in a slurry that can be deposited on a semiconductor substrate or device by a spin coater. The glass coated device is soft baked in a oven before being patterned by standard photolithography techniques. Next, the device is placed in an oxygen plasma to burn off the photoresist which leaves only the patterned fritted glass over the device. To actually fuse the glass together, the device is slowly pushed into a firing furnace.
Unfortunately, it was discovered that the firing furnace did not have good temperature uniformity from substrate to substrate or within a substrate. The firing furnace required about 15 minutes for temperature equilibrium after the substrates were inserted. The zone at the mouth of the furance recovered more slowly than the other zones, so that there is a longitudinal temperature gradient. Reducing the gradient by increasing firing time caused degradation due to overfiring. Overfiring will cause soft, low breakdown voltage and high leakage at 150 degrees Celsius. Underfiring of the glass will give an incomplete seal leading to low breakdown and high collector-base leakage.
To appreciate the seriousness of the uniformity problem, one must realize that there are approximately 45 three inch wafers or substrates or approximately 25 five inch wafers loaded into a firing furnace at one time. On each three inch wafer there may be as many as 1,150 dice or as many as 4,600 dice on a single five inch wafer. Underfiring would occur at the load end of the furance where the temperature would not be high enough to completely fuse the glass. Overfiring would occur in the source end of the furnace where the temperature would be too high. Hundreds to thousands of dice would be rejected due to improper firing of the fritted glass.